The present invention relates to a method of observing a specimen using a scanning electron microscope for reviewing defects on the surfaces of a semiconductor electronic circuit substrate, a liquid crystal display substrate, etc.
With the objective of determining in detail each defect produced on a semiconductor electronic circuit substrate, automatic defect review (ADR: Automatic Defect Review) has been carried out for re-detecting an image at each defect position, detected by an optical inspecting device or an electron beam inspecting device, with high resolution by use of an electron beam microscope. The automatic defect review involves the collection of images of a large number of defects that are identified by the inspecting device automatically and at high speed.
As an example of such automatic defect review, reference is made, for example, to Japanese Patent Laid-open No. 2002-124555 and Japanese Patent Laid-open No. 2003-98114.
In the automatic defect review that is carried out using an electron beam microscope, much of the processing time is spent on automatic focusing. Thus, there has been a strong demand for shortening the time necessary for automatic focusing.
One example of an image detecting sequence that may be used during automatic defect review is shown in FIG. 8. In FIG. 8, reference numeral 80 indicates defect position data. Prior to the review, coordinate positions on a wafer, where respective defects have been detected by an optical or electron beam inspecting device, are inputted to a review electron beam microscope as the defect position data 80.
Next, a focus map creating process 81 is performed prior to the picking-up of images of the respective defects. A focus map is equivalent to one in which a wafer in-plane distribution of focus positions caused by warpage of a wafer surface and electrostatic charging of the surface or the like has been estimated. Focus positions are normally determined at a plurality of points (about several to ten points or more, which are not necessarily limited to defects) by manual control, and a curved surface sufficient to produce a good approximation of the result of measurement of the focus positions is estimated. By schematically estimating the focus positions in advance, the focus-position searching range at each defect position can be narrowed. As a result, the time required to perform the required focusing can be shortened. Thus, an improvement in the accuracy of estimation of each focus position using the focus map is important to the shortening of the automatic focusing time.
After the focus map has been determined, the stage carrying the specimen is moved to each position intended for detection (82), where image detection is performed. At the time of image detection, images are picked up at two types of resolution, including low and high resolutions. Further, images are picked up at two defect and reference points with respect to the respective resolutions. That is, the images of four types (low resolution reference, low resolution defect, high resolution reference and high resolution defect) are detected.
The reason why image detecting is performed at both low and high resolutions is as follows. The accuracy of defect position data outputted by the inspecting device might reach about 20 mm according to circumstances. In such a case, there is a need to detect an image with a wide field of view (at a low resolution) for the purpose of detecting a defect within a field of view. On the other hand, the wide-field (low resolution) image might often be insufficient or short in resolution as a review application. Therefore, two-stage imaging is carried out, which sufficiently picks up a wide-field low resolution image, detects a defect within a low resolution image field of view, specifies a defect position in this condition, and images the neighborhood of the defect position at a high resolution.
The reason why both defect and reference images are picked up is as follows. In addition to the defect image, each of the images at the same points of chips that are positioned adjacent to each other is detected as a reference image. Consequentially, a user is able to observe the defect image and the reference image by comparison and use the result to provide an understanding of a defect section. This can also be used in ADC (Automatic Defect Classification).
Returning to the description of FIG. 8, the picking up of the above-described four types of images is executed sequentially while the stage is being moved. That is, the stage is moved to a reference position (82), where a low resolution reference image is picked up (83). Next, the stage is moved (84) to detect a low resolution defect image. Then, a defect position in the corresponding low resolution image is specified based on the low resolution reference image and the low resolution defect image (86). The periphery of the specified defect position is imaged at a high resolution (87). Next, the stage is moved (88) to detect a reference high resolution image (89).
Now, focusing is performed every time, upon picking up the above-described four types of images. The contents of automatic focusing processing will be explained below the diagrams using 831, 832 and 833 in FIG. 8.
Upon automatic focusing at each point, an estimated value Zest of a focus position at each position to be detected is determined using the focus map (81) obtained in advance (831). Next, images at a different focus position are picked up while the focus position is being changed in the vicinity of the estimated value Zest (832). That is, images are sequentially picked up while the focus position is being changed to Z1, Z2, Z3, . . . with Z1<Z2<Z3 . . . <Zest . . . <Zn−2<Zn−1<Zn. Next, focus measures are calculated from the detected images. The focus measure is equivalent to an index obtained by quantifying focus matching. Various definitions are known therefor. A focus measure is normally defined in such a manner that the value thereof increases as it approaches a focused state. There is, for example, an amount obtained by integrating the intensities of the absolute values of differential operator's outputs over the entire image, etc. Reference numeral 833 indicates a result obtained by plotting focusing or focus measures determined with respect to the images picked up while the focus position is being changed. In the result 833, the position where the focus measure reaches the maximum can be regarded as a focus position.
The time necessary for automatic focusing at the time of image pickup is proportional to the number of pickups of images having a different focus position. Alternatively, the time is also proportional to the area of each image detection region. Thus, a reduction in the number of images to be picked up and a narrowing of the detection region are important to any shortening of the automatic focusing time.